Organic zinc salts are used in rubber compounding (e.g., in the manufacture of tires). When a rubber product is mixed and formed (e.g., in a mold), it is typically heated in a curing process (i.e., for a length of time and/or at a temperature sufficient to effect such curing). Careful control of process conditions in such a curing process is essential to impart one or more desired characteristics to a cured rubber product. However, in the case of large and/or thick rubber products, heat transfer to the rubber product during a curing may be non-uniform throughout the rubber material due to the heat transfer characteristics of the material being cured (i.e., longer times are necessary for heat transfer to the bulk material from the surface of a mold). This may result in an uneven curing process (e.g., over-cured surface regions relative to the bulk material), yielding a product with undesirable characteristics resulting from such over-curing at a surface of a rubber product, a phenomenon commonly referred to as reversion. Such an over-cured surface region may have a lower hardness, elastic modulus, abrasion resistance, etc.
There are a number of additives available to reduce the occurrence and/or mitigate the extent of reversion in the curing of rubber products. U.S. Pat. Nos. 5,302,315, 5,610,240, 5,623,007 and 5,872,188 (the relevant portions of each of which are incorporated herein by reference) all address either the preparation or use of such anti-reversion agents. Such anti-reversion agents may comprise a mixture of organic zinc salts (also known as organic zinc soaps) prepared from C7-C24 fatty acid and zinc salts of aromatic carboxylic acids such as benzoic acid, chlorobenzoic acid, and/or methylbenzoic acid. Together with conventional organic zinc salts such as zinc stearate, zinc laurate, and zinc oleate, these organic zinc salts have large markets and varied applications in the rubber, plastics and cosmetics industries.
One method for preparing such organic zinc salts is referred to as a fusion method. In a fusion method for preparing such organic zinc salts, a zinc compound such as zinc oxide, zinc hydroxide, and/or zinc carbonate is heated in the presence of an organic acid such as stearic acid, oleic acid, and/or other organic acid to yield the corresponding organic zinc salt. Organic zinc salts formed via such a fusion method have a higher bulk density and a larger volume-to-mass (V/m) ratio relative to organic zinc salts prepared via other methods.
A second method for preparing such organic zinc salts employs the reaction between a sodium, ammonium or potassium salt of one or more organic acids and zinc sulfate, zinc acetate and/or zinc chloride. The product of such a method may be precipitated out of an aqueous reaction medium, and washed with water to remove byproducts such as sodium sulfate, sodium chloride, ammonium sulfate, etc., and then dried. Organic zinc salts produced by this second method are typically referred to as precipitated organic zinc salts, and are considered superior relative to similar salts produced via the fusion method previously described.
However, this second organic zinc salt production methodology requires additional process steps and/or chemical reagents relative to a fusion method to effect the desired transformation, resulting in an increased cost. This second method also produces additional waste in the form of byproducts such as sodium sulfate, ammonium sulfate, and/or sodium chloride. Such sodium sulfate, ammonium sulfate, and/or sodium chloride byproducts are often in such a dilute state after washing a product organic zinc salt that it is not economical to recover such byproducts, and the filtrate is discarded. Disposal of such filtrate (with dissolved byproducts therein) has potentially negative environmental consequences via an increase in the level of water soluble salts in the environment.
A third method of making organic zinc salts comprising dithiocarbamates is described in U.S. Pat. No. 2,492,314 (the relevant portions of which are incorporated herein by reference). An amine, carbon disulfide, and zinc oxide are employed to produce organic zinc salts under anhydrous conditions. However, the resulting product has less than desirable properties, including relatively coarse, large particles. Thus, the preferred method for producing organic zinc salts has been the second method described above, from water soluble zinc salts like zinc chloride, and/or zinc acetate, and more preferably, zinc sulfate, as described in U.S. Pat. No. 6,534,675 (the relevant portions of which are incorporated herein by reference).
It is known that zinc-ammonia-carbonate complexes and solutions thereof may used to produce active zinc oxide and/or zinc carbonate (see, e.g., U.S. Pat. No. 6,555,075, the relevant portions of which are incorporated herein by reference). The active zinc oxide or zinc carbonate produced thereby is then further reacted with one or more organic acids to make the corresponding organic zinc salts. This present invention improves upon the processes described above, providing methods whereby organic zinc salts may be obtained directly from a zinc-ammonia-carbonate complex and/or a solution thereof, obviating the need for a dedicated step to generate an active zinc oxide and/or zinc carbonate. The organic zinc salts afforded by the present methods may also possess superior characteristics relative to organic zinc salts produced by the fusion method described above.
Organic salts may also used to coat siliceous particles like precipitated silica, clay, kaolin, talc, aluminum silicate, calcium carbonate, carbon black and mixtures thereof. Such coated particles, when added to rubber compositions as fillers, impart advantageous characteristics to rubber products formed therefrom, including improved abrasion resistance, modulus, and tensile strength (see, e.g., U.S. Pat. Appl. Publ. Nos. 2006/0281009, 2008/0194748, and 2008/0161475, and U.S. Pat. Nos. 6,291,572 and 6,333,375, the relevant portions of each of which are incorporated herein by reference).
Organic zinc salts and organic zinc salt coated fillers may improve the Mooney viscosity, modulus, hardness and abrasion resistance of a rubber compound compared with untreated fillers. Reducing the Mooney viscosity results in a more facile processing of a rubber compounding composition containing organic zinc salts and/or organic zinc salt coated fillers. This improvement is important for tires. An organic zinc salt coating can be formed on siliceous fillers when such fillers are first precipitated from solution, before drying, obviating a need for multiple drying steps. Of course, the zinc salts or mixture of zinc salts of this invention can be added separately with siliceous fillers, rubber processing aids such as accelerators, sulfur, antioxidants, processing oils, etc., with or without silane in rubber recipes to obtain the desired properties as the treated fillers above.